1. Field of the Invention
This invention relates to a cathode ray tube, and more particularly to a cathode ray tube having a fluorescent screen of a copper and aluminum activated zinc sulfide phosphor.
2. Description of the Prior Art
As a green emitting phosphor of a fluorescent screen of a color television cathode ray tube (hereinafter referred to as "color TV CRT"), a copper and aluminum activated zinc sulfide phosphor (ZnS:Cu,Al) has heretofore been employed. The ZnS:Cu,Al phosphor has an emission color point within a green region in the color coordinates of CIE color system represented by values x and y ranging from 0.250 to 0.285 and 0.578 to 0.653, respectively.
Recently, there has been a demand for a color TV CRT having high white luminance. To meet the demand, there has been employed a green emitting phosphor having an emission color point within a yellowish green region which is biased to a longer wavelength side than the above-mentioned green region. More concretely, a green emitting phosphor which exhibits yellowish green emission the value x of the color point of which is not less than 0.295 has been employed. As described hereinbelow, the conventional ZnS:Cu,Al phosphor does not exhibit yellowish green emission as mentioned above. Accordingly, as the green emitting phosphor exhibiting a yellowish green emission, there has been employed a mixed phosphor in which a proper amount of the ZnS:Cu,Al green emitting phosphor is mixed with a proper amount of a gold and aluminum activated zinc sulfide yellow emitting phosphor (ZnS:Au,Al) so as to obtain yellowish green emission, or employed a gold, copper and aluminum activated zinc sulfide phosphor (ZnS:Au,Cu,Al) which solely exhibits yellowish green emission.
However, the thermal stability of the abovementioned ZnS:Au,Al and ZnS:Au,Cu,Al phosphors is low. Therefore, when these phosphors are heated to an ordinary baking temperature for the production of a fluorescent screen of a color TV CRT (450.degree.-480.degree. C.), the emission luminance thereof is lowered and the emission spectra thereof are shifted to a shorter wavelength side. Accordingly, when the fluorescent screen employing the above-mentioned mixed phosphor or ZnS:Au,Cu,Al phosphor is prepared, baking must be carried out for a long period of time at a temperature lower than the ordinary baking temperature. Such circumstances result in lowering of operating efficiency and increase in manufacturing cost. Further, since the ZnS:Au,Al phosphor and the ZnS:Au,Cu,Al phosphor need to be fired at a high temperature for a long period of time during the preparation thereof, and besides they contain expensive Au as an activator, the manufacturing cost of these phosphors is very high in comparison with the ZnS:Cu,Al phosphor. Under the above-mentioned circumstances, there is a demand for a green emitting phosphor for a color TV CRT which exhibits yellowish green emission and does not have the above-mentioned defects, and accordingly, can be employed in place of the above-mentioned mixed phosphor or ZnS:Au,Cu,Al phosphor.
ZnS:Cu,Al phosphors do not have the above-mentioned defects of the ZnS:Au,Al phosphor and the ZnS:Au,Cu,Al phosphor. That is, the ZnS:Cu,Al phosphor is stable at the ordinary baking temperature, and therefore, when heated to the temperature, the emission luminance thereof is not lowered and the emission color thereof is not changed. Further, the ZnS:Cu,Al phosphor can easily be prepared in comparison with the ZnS:Au,Al phosphor and the ZnS:Au,Cu,Al phosphor. Furthermore, the ZnS:Cu,Al phosphor is more inexpensive than the ZnS:Au,Al phosphor and the ZnS:Au,Cu,Al phosphor because it does not contain Au. From the above viewpoints, it is desirable to employ the ZnS:Cu,Al phosphor in a color TV CRT having high white luminance in place of the above-mentioned mixed phosphor or ZnS:Au,Cu,Al phosphor. However, as described hereinbelow in detail, the conventional ZnS:Cu,Al phosphor does not exhibit yellowish green emission, and accordingly, there is a great demand for a ZnS:Cu,Al phosphor which exhibits yellowish green emission.
As a phosphor exhibiting yellowish green emission, has been known a copper and aluminum activated zinc cadmium sulfide phosphor [(Zn,Cd)S:Cu,Al]which was practically used as a green emitting phosphor of a color TV CRT before the practical use of the conventional ZnS:Cu,Al phosphor. However, since the (Zn,Cd)S:Cu,Al phosphor contains cadmium which is very toxic to the human body, it is desirable not to practically use the (Zn,Cd)S:Cu,Al phosphor from the viewpoint of prevention of environmental pollution.
The conventional ZnS:Cu,Al phosphor can be prepared by mixing a proper amount of raw materials of Cu and Au activators with a powdery zinc sulfide (ZnS) precipitated from a zinc ion-containing aqueous solution to obtain a raw material mixture, and then firing the raw material mixture in a reducing atmosphere such as a carbon vapor atmosphere and a hydrogen sulfide atmosphere at a temperature within the range of 800.degree. to 1200.degree. C. Generally, a proper amount of sulfur is added to the raw material mixture in order to prevent oxidization of the ZnS. Further, a proper amount of flux is generally added to the raw material mixture in order to, for example, enhance the emission luminance of the ZnS:Cu,Al phosphor obtained. The emission color of the ZnS:Cu,Al phosphor prepared in accordance with the above-mentioned process depends upon the Cu activator value. However, no matter how the Cu activator value may be altered, value x of the emission color point of the ZnS:Cu,Al phosphor obtained is not more than 0.285. Accordingly, a ZnS:Cu,Al phosphor which exhibits yellowish green emission the value x of the color point of which is not less than 0.295 cannot be prepared in accordance with the above-mentioned process. In FIG. 1, curves c and d show the relationships between the Cu activator value of the conventional ZnS:Cu,Al phosphor and the values x and y of the emission color point of the phosphor, respectively. As is clear from the curve c, when the Cu activator value is less than about 2.times.10.sup.-4 grams per 1 gram of ZnS (Similarly, the Cu activator value and Al activator value described hereinafter are represented by the number of grams per 1 gram of ZnS), value x of the emission color point increases as the Cu activator value increases. When the Cu activator value is about 2.times.10.sup.-4 g/g, value x becomes maximum (x=0.285), and when the Cu activator value is more than about 2.times.10.sup.-4 g/g, value x gradually decreases as the Cu activator value increases. On the other hand, as is clear from the curve d, value y of the emission color point gradually decreases as the Cu activator value increases. In the region extending from green to yellow of the color coordinates of CIE color system, the change of value x much more contributes to the change of color than the change of value y. Therefore, the emission color of the conventional ZnS:Cu,Al phosphor gradually shifts to a longer wavelength side as the Cu activator value increases when it is less than about 2.times.10.sup.-4 g/g, have the longest wavelength when the Cu activator value is about 2.times.10.sup.-4 g/g, and gradually shifts to a shorter wavelength side as the Cu activator value increases when it is more than about 2.times.10.sup.-4 g/g. As described above, the conventional ZnS:Cu,Al phosphor has an emission color point within a green region having value x of not more than 0.285. Accordingly, value x of more than 0.285 cannot be attained by the conventional ZnS:Cu,Al phosphor.
The Cu activator value also has an effect upon the emission luminance of the ZnS:Cu,Al phosphor. In FIG. 2, curve b shows the relationship between the Cu activator value of the conventional ZnS:Cu,Al phosphor and the emission luminance of the phosphor. As is clear from the curve b, when the Cu activator value is not less than 10.sup.-4 g/g, the emission luminance lowers as the Cu activator value increases. In particular, when the Cu activator value becomes not less than 5.times.10.sup.-4 g/g, the emission luminance falls prominently. Since the body color of the ZnS:Cu,Al phosphor changes from white to gray as the Cu activator value increases, it is consider that the above fall of the emission luminance is based on the fact as follows. That is, as the Cu activator value increases, the amount of Cu not taken into the ZnS host material crystal increases and there is deposited on the surface of the phosphor particles a black copper sulfide which absorbs the light emitted from the phosphor. In FIG. 3, curves c and d show the relationships between the Cu activator value of the conventional ZnS:Cu,Al phosphor and the values x and y of the color point of body color of the phosphor, respectively. Further, in FIG. 4, curve b shows the relationship between the Cu activator value of the conventional ZnS:Cu,Al phosphor and the mean reflectance in the visible spectrum region of the phosphor. As is clear from curves c and d in FIG. 3, the color point of the body color of the conventional ZnS:Cu,Al phosphor lies within a white region in the color coordinates of CIE color system. Further, as is clear from the curve b in FIG. 4, the reflectance of the conventional ZnS:Cu,Al phosphor gradually lowers as the Cu activator value increases. That is, the conventional ZnS:Cu,Al phosphor has a white body color when the Cu activator value is small. However, as the Cu activator valve increases, the body color gradually changes from white to gray and the reflectance of the phosphor gradually lowers.
Based on the relationship between the Cu activator value of the conventional ZnS:Cu,Al phosphor and the emission color of the phosphor (curves c and d in FIG. 1), and the relationship between the Cu activator value of the conventional ZnS:Cu,Al phosphor and the emission luminance of the phosphor (curve b in FIG. 2) as described above, the ZnS:Cu,Al green emitting phosphor in which the Cu activator value is within the range of 10.sup.-5 to 3.times.10.sup.-4 g/g, and values x and y are within the ranges 0.25 to 0.285 and 0.578 to 0.653, respectively, has heretofore been practically used as a green emitting phosphor of a fluorescent screen of a color TV CRT. Such a ZnS:Cu,Al phosphor, for example, is disclosed in U.S. Pat. No. 4,038,205. That is, the patent discloses ZnS:Cu,Al green emitting phosphor employed as a green emitting phosphor of a fluorescent screen of a color TV CRT in which the Cu activator value is within the range of 10.sup.-5 to 2.times.10.sup.-4 g/g, and values x and y are within the ranges of 0.250 to 0.280 and 0.560 to 0.615, respectively. In the conventional ZnS:Cu,Al phosphor, the Al activator value is generally 1/2 to 7 times as large as the Cu activator value.
As explained above, the conventional ZnS:Cu,Al phosphor does not exhibit yellowish green emission. However, as described above, the ZnS:Cu,Al phosphor is more stable at a baking temperature usually employed in production of a fluorescent screen of a color TV CRT and is more inexpensive than the above-mentioned mixed phosphor or ZnS:Au,Cu,Al phosphor which exhibits yellowish green emission and at present, is practically used as a green emitting phosphor for a color TV CRT having high white luminance. Accordingly, there is a great demand for ZnS:Cu,Al phosphor which exhibits yellowish green emission of high luminance. Further, there is a great demand for a color TV CRT having a fluorescent screen in which such a ZnS:Cu,Al phosphor is employed as a green emitting phosphor.